A first look at the submillimeter Sun with ALMA

We present the first full-disk solar images obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) in Band 7 (0.86 mm; 347 GHz). In spite of the low spatial resolution (21"), several interesting results were obtained. During our observation, the sun was practically devoid of active regions. Quiet Sun structures on the disk are similar to those in Atmospheric Imaging Assembly images at 1600 angstrom and 304 angstrom, after the latter are smoothed to the ALMA resolution, as noted previously for Band 6 (1.26 mm) and Band 3 (3 mm) images; they are also similar to negative Ha images of equivalent resolution. Polar coronal holes, which are clearly seen in the 304 angstrom band and small H alpha filaments, are not detectable at 0.86 mm We computed the center-to-limb variation of the brightness temperature, T-b, in Band 7, as well as in Bands 6 and 3, which were obtained during the same campaign, and we combined them to a unique curve of T-b(log mu(100)), where mu(100) is the cosine of the heliocentric angle reduced to 100 GHz. Assuming that the absolute calibration of the Band 3 commissioning observations is accurate, we deduced a brightness temperature at the center of the disk of 6085 K for Band 7, instead of the value of 5500 K, extrapolated from the recommended values for Bands 3 and 6. More importantly, the T-b(log mu(100)) curve flattens at large values of mu(100), and so does the corresponding T-e(log tau(100)) at large tau(100). This is probably an indication that we are approaching the temperature minimum.